SafeGrid Tutorial · The following tutorial is designed to help users of SafeGrid Earthing Software...

SafeGrid Tutorial – Telecomm Mobile Repeater Station Electrotechnik Pty Ltd www.elek.com.au

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SafeGrid Tutorial

Telecomm Mobile Repeater Station – Use of Rods to e nsure R grid ≤ 5 Ω

The following tutorial is designed to help users of SafeGrid Earthing Software to learn about and practice the application of powerful operating tools and principles. This tutorial and the contained images were produced using SafeGrid Version 2.5 however the same principles may be applicable and similar functions available for other older or newer versions also.

Contents

1. Mobile phone repeater station earthing arrangement ........................................... 2

1.1 Purpose of the earthing system ............................................................................. 2

2. Modelling the earth grid in SafeGrid ...................................................................... 3

3. Soil resistivity modelling ........................................................................................... 4

4. SafeGrid Results - calculated grid resistance.......................................................... 5

4.1 Plot of grid resistance versus no. of rods ............................................................. 6

5. Conclusion – No. of rods required ........................................................................... 6

Appendix - SafeGrid design report for 4 rods case ....................................................... 7


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1. Mobile phone repeater station earthing arrangement The mobile phone repeater station earthing consists of interconnected grid conductors covering the lease area of 9 m by 14.5 m of conductors buried at a depth of 0.5 m. The grid is buried at separation distances of 0.5 m and 1 m from the perimeter fence and the shelter building respectively. There are three 1 m2 buried plates underneath the concrete pad footing for each leg of the antenna structure. Rods of 12 m lengths are added as required to reduce the overall earthing resistance.

Figure 1 Sketch of mobile phone repeater station earthing system

1.1 Purpose of the earthing system

The purpose of the earthing system is to provide a low impedance path to the earth from personnel and equipment protection during lightning strikes. The customer has specified that the total earth grid impedance must not exceed 5 Ω.


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2. Modelling the earth grid in SafeGrid The earthing grid was drawn using CAD software and imported into SafeGrid as a DXF file. Figure 2 shows the CAD model for earthing arrangement. The model includes the metal plates which were modelled as tightly meshed straight line connected segments.

Figure 2 Model of earth grid drawn in CAD and saved as DXF file for importing into SafeGrid

Figure 3 DXF CAD file imported into SafeGrid


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3. Soil resistivity modelling Soil resistivity measurements were taken in the field using the Wenner Array Method at electrode spacing’s of between 1 m and 8 m due to space limitations. Two sets of measurements were taken in different directions. Note it is good practice to take at least 5 measurements per set.

Figure 4 Wenner field resistivity measurements

The calculated 2-layer soil model using E-W measurements was as follows (Figure 5): Top layer resistivity = 6390 Ω-m Depth of top layer = 3.13 m Bottom layer resistivity = 139.3 Ω-m Goodness of fit = 0.928 (very good)

Figure 5 E-W measurements modelled in SafeGrid LM-RES module


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4. SafeGrid Results - calculated grid resistance Grid resistance was calculated for the various cases as follows: Grid model Surface potentials plot Case Grid resistance (Ω)

XY

No rods 128.93

3D

1 rod 13.8

3D

2 rods 7.9

3D

3 rods 5.9

3D

4 rods 4.82

3D

5 rods 4.31


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4.1 Plot of grid resistance versus no. of rods Adding rods is very effective at reducing the grid resistance. This is because the rods are penetrating the relatively low resistivity bottom soil layer.

5. Conclusion – No. of rods required In conclusion adding 4 rods to the grid at the corners is enough to reduce the grid resistance to below 5 Ω.


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Appendix - SafeGrid design report for 4 rods case Refer to the attached sheets for a demonstration of the output results from SafeGrid.

SafeGrid Earthing Software complies with IEEE Std 80 and IEC 60479.

Project:SafeGrid Tutorial - Telecomm Mobile Repeater StationScenario:4 Rods Case

Design Inputs

Table 1: Soil characteristics

Refer to separate page for LM-RES soil modelling.

Table 2: Grid energisation

Table 3: Grid and rod conductor size

Table 4: Measurement units

Table 5: System

Model Determine values using soil modellingprogram (S-RES)

Top layer resistivity (Ohm.m) 6390.243Top layer depth (m) 3.135Bottom layer resistivity (Ohm.m) 139.301

Units Current (A)Magnitude 10

Method Specify grid conductor radiusConductor radius (mm) 10

Units Metric

Frequency (Hz) 50

SafeGrid V2.5 Design Report 7/05/2013, 5:00 PM

SafeGrid Earthing Design Software Electrotechnik Pty Ltd -www.ELEK.com.au Page 1 of 11

http://www.elek.com.au






Levenberg-Marquardt Soil Resistivity (LM-RES) Modelling

Table 6: User Inputs

Table 7: Field measurements

Table 8: Results

Figure 1: Non-linear model fit to field measurements

Soil model Horizontal 2-LayerMeasurement method WennerMeasurement values Resistance (Ohms)Account for probe depth? (True/False) FALSEDriven depth at short spacings (m) -Remove outliers? (True/False) FALSE

Spacing (m) Resistance (Ohms) Included?(True/False)

R1 1 900 TRUER2 2 521 TRUER3 4 122.7 TRUER4 8 19.7 TRUE

Top layer soil resistivity (Ohm.m) 6390.242856Top layer depth (m) 3.13451Bottom layer soil resistivity (Ohm.m) 139.30134Reflection factor, k -0.957332Goodness of Fit (R-square) 0.927956








Buried Grid Model

Table 9: Segments

Figure 2: Buried grid preview

No. of segments accounting forintersections only 660

No. of segments after segmentation (foraccuracy) 677

Fault location (segment no.) 1Total length of conductor network (m) 179.5








Safety Criteria (ISAFE) Calculation

Table 10: User Inputs

Table 11: Results

Safety standard (body resistance) IEEEFibrillation current method 50kg - IEEEBody resistance curve (IEC) -Conditions (IEC) -Foot resistance calculation method IEEE80:2000Additional series resistance (i.e. shoe orglove) (Ohms) 0

Fault clearing time (s) 0.5System frequency (Hz) 50System X/R ratio 20Decrement factor DefaultDecrement factor value 1.061755

Sub-surface layer resistivity (Ohm.m) 6390.242856Use top layer soil resistivity? (True/False) TRUESurface material resistivity (Ohm.m) 500Surface layer depth (m) 0.18

Allowable Touch Voltage Limit (V) 254.184869Allowable Step Voltage Limit (V) 553.217983Permissible body current (A) 0.164049Body resistance - 1 hand to both feet(Ohms) 1000

Body resistance - 1 foot to other foot(Ohms) 1000








Method of Images Grounding (MOI-GND) Modelling

Table 12: Results

Table 13: Summary of Inputs

Grid Impedance (Ohms) 4.825Grid Potential Rise, GPR (V) 48.246

Top soil layer resistivity (Ohm.m) 6390.24Depth top soil layer (m) 3.13Bottom soil layer resistivity (Ohm.m) 139.3Excitation current (A) 10Faulted segment no. 1Final no. of segments 677Conductor radius (m) 0.01Conductivity of buried conductor (S/m) 57E+6Frequency (Hz) 50Calculation delta 0.001








Figure 3: Surface potentials (V) - 3D view

Figure 4: Surface potentials - Colour map








Figure 5: Surface potentials (V) - X-Y view

Figure 6: Surface potentials - Colour map








Figure 7: Touch potentials (V) - 3D view

Figure 8: Touch potentials - Colour map








Figure 9: Touch potentials (V) - X-Y view

Figure 10: Touch potentials - Colour map








Figure 11: Step potentials (V) - 3D view

Figure 12: Step potentials - Colour map








Figure 13: Step potentials (V) - X-Y view

Figure 14: Step potentials - Colour map








SafeGrid Tutorial · The following tutorial is designed to help users of SafeGrid Earthing Software...

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